1. Field of the Invention
The present invention relates to optical scanners, and particularly to a semi-retroreflective system. The invention further relates to optical scanners capable of calculating the distance to a target and optical scanners arranged to overcome the problem of specular reflection "blinding".
2. Discussion of the Prior Art
In optical scanning systems (for example for scanning bar code symbols) a light beam is scanned across an indicia to be read, and reflected light is collected, using either retroreflective or non-retroreflective collection optics.
In retroreflective light collection, a single optical component, e.g. a rotating mirror, as described in Krichever et al. U.S. Pat. No. 4,816,661 or Shepard et al. U.S. Pat. No. 4,409,470, both herein incorporated by reference, scans the beam across a target surface and directs the collected light to a detector. The rotating mirror must be relatively large to receive the incoming light, but only a small detector is required because the rotating mirror can focus the light on to a small area. As a result, the detector has a relatively small field of view, which increases signal-to-noise ratio. Of course, small scan elements are preferable because of the reduced energy consumption and increased frequency response. When the scan element becomes sufficiently small, however, the area of the scanning mirror can no longer be used as the aperture for the received light. One solution is to use a staring detection system (a non-retroreflective system) which receives a light signal from the entire field which the scanned laser spot covers.
In non-retroreflective light collection, the reflected laser light is not collected by the same rotating mirror used for scanning. Instead, the detector is constructed to have a large field of view so that the reflected laser light traces across the surface of the detector. Because the rotating mirror need only handle the outgoing light beam, it can be made much smaller. But the detector must be relatively large in order to receive the incoming light beam from all scanned locations. This arrangement necessarily means a low signal-to-noise ratio.
It has become increasingly desirable not only to increase the scanning rate of bar code scanners (e.g., to increase the throughput of scanned items), but also to increase the signal-to-noise ratio of bar code scanners to accommodate the increasing information density stored in bar code labels.
In recent years, there have been increasing demands for storing more information in a bar code label. In particular, there is a desire to create a "portable data file", as opposed to the "license plate" nature of conventional bar code symbols. For example, a typical UPC bar code only contains the manufacturer's code and the item number. The price look-up must be accessed in a database keyed to the item number in the bar code. Alternatively, a two-dimensional bar code could be used to store all of the relevant information, such as price, the name of the product, manufacturer, weight, inventory data, and expiration date. This would constitute a "portable data file" because the information could be retrieved without access to a database. While a price look-up database file is conveniently available in a retail environment, this may not be the case in other applications.
Although two-dimensional bar code schemes do not have vertical redundancy, they incorporate other forms of redundancy as well as other advantages, while increasing the density of information storage. For example, Code PDF417 allows stitching of partial scans, error detection, and error correction.
Our co-pending application Ser. No. 08/268,360, filed Jun. 30, 1994, and commonly assigned with the present application, describes a system which satisfies the seemingly conflicting requirements, encountered in conventional retroreflecting and non-retroreflecting scanning schemes, of both a large working range and high scan speeds. It does, however, require the use of a two-dimensional array of individual light detection elements, which may not always be desirable for reasons of cost.
Hand-held solid state scanners do not have any built-in device or technique for estimating the distance to a target as a result of which such scanners have a limited depth of field compared to laser scanners. Although the depth of field can be increased by use of a high quality optical system the cost of the scanner is thus considerably increased.
Known scanners, and in particular omni-directional scanners suffer from a further problem of undesirable specular reflection caused by the undesirable deflection of a beam reflected from an indicia to be read onto a photodetector.